1. FIELD OF THE INVENTION
The invention relates to a chain lever hoist (hereafter "lever hoist") with a load chain and that can be manually operated by means of a lever, for example, to wind goods up or down or to pull them along.
2. DESCRIPTION OF THE PRIOR ART
Generally speaking, this kind of lever hoist must not only to be capable of winding goods up and down (hereafter "winding operation") by means of a lever-operated load chain, but must also allow the chain to move freely under no-load conditions. In other words, it is normally held to be essential that the chain hoist should be provided with what is sometimes called a "load sheave" to allow for the free running of the chain.
Of the conventional types of lever hoist in current use, one of the best known and most commonly used types is structured such that the load sheave, which is fitted to the main framework of the apparatus, is also joined to a spindle to which a fixed friction plate is secured, said spindle also being screwed into a hub incorporating a device for switching between upward and downward winding operations, the part of the spindle between the fixed friction plate and the hub being also fitted with a ratchet gear which has brake linings on both sides and which is able to slide and rotate freely on said spindle such that the distance between said fixed friction plate and said hub can be varied by screwing the hub up or down the spindle in such a way as to squeeze or release said ratchet gear and brake linings, such action being also assisted by the fitting, for example, of a coil spring in extended condition between said fixed friction plate and said hub such that the hub is ordinarily pressed outwards by the force of the coil spring, thereby easing the contact pressure of the hub on said brake linings and, in so doing, preventing the brake from being applied. In lever hoists of the type described above, it is common for a heavy duty hoist with a load capacity of 0.5 tons or more to have its load sheave and spindle linked through the medium of a plurality of reduction gears but for a light hoist with a load capacity of less than 0.5 tons to have its load sheave and spindle connected to each other directly. Spring lever hoists of the type outlined above suffer from a significant drawback, however, in that when the chain is moved quickly while the load sheave is in free running operation, the spindle turns but the hub does not turn with it and, since the spindle and the hub are linked by a threaded connection, the space between the fixed friction plate and the hub is automatically narrowed and the brake applied, thereby eliminating the capacity for free movement. The hub thus has to be rotated manually back each time this happens in order to re-open the gap between the hub and the fixed friction plate and release the brake.
Another problem with the conventional type of spring lever hoist is that, when winding down a light object, the force with which the spindle is being screwed into the hub is sometimes weaker than that with which the coil spring is pressing the hub outwards. In this sort of case, the object being lowered is sometimes let down too quickly and this has in the past led to accidents, some of which have been fatal. In other words, conventional spring lever hoists have what we might call a reciprocal problem in that, if the coil spring is fairly powerfully extended, this will ensure that there is plenty of play in the hub and spindle but there will also be a risk that light objects may be lowered too quickly, leading to accidents. On the other hand, if the spring is only weakly extended, this will help prevent accidents when lowering light objects but, conversely, any rapid movement of the chain while running freely over the load sheave will immediately cause the brake to operate, thus interfering with the free movement of the chain. There are, of course, a variety of mechanisms that can be used to ensure the free running of a load sheave. These include a mechanism whereby, in lever hoists equipped with reduction gears of the type referred to above, the reduction gear spindle can be caused to slide as a means of shifting the gear teeth out of line with each other, thereby permitting the load sheave to rotate freely. Another such mechanism disconnects the pawl from the ratchet gear and this again has the effect of allowing the load sheave to rotate freely. The use of these types of mechanism certainly helps prevent the sorts of problems outlined above but, at the same time, the complexity of these mechanisms can in itself be a source of problems in that the smooth operation of the apparatus is rendered more problematic. There is also a concomitant loss of reliability in that the apparatus tends to break down more often. Moreover, the change from a free running to a winding action always requires a single action.